feat($rootScope): implement $applyAsync to support combining calls to $apply into a single digest.

It is now possible to queue up multiple expressions to be evaluated in a single digest using
$applyAsync. The asynchronous expressions will be evaluated either 1) the next time $apply or
$rootScope.$digest is called, or 2) after after the queue flushing scheduled for the next turn
occurs (roughly ~10ms depending on browser and application).